The transcription of the first verse of the Berta song above (copied from the linked site) is not actually right. If you listen to the vid (and having an idea what you're listening for probably helps) you can see what they are singing is:

Ain’t that Berta comin, down that road, well.
She walk like Berta but she, walk too slow, well.
She walk too slow baby, she walk too slow, well.
She walk like Berta but she, she walk too slow, well.

The verse is obviously a variant of what is also found as the first verse of Tommy Johnson's Maggie Campbell blues:

Who's that yonder coming down the road? coming down the road?
Who that yonder coming down the road?
Looks like Maggie baby, but she walks too slow.

I like the Tommy Johnson verse and even wrote a song riffing on this idea but I had not heard this alternative take before.